module LK
    implicit none
    real,save :: k1,k2,k3,k4
    subroutine kd4(y1,y2,x,n,f1,f2)
        implicit none
        real :: y1(),y2(),x(),dy1,dy2
        real,external :: f1,f2
        dy1=f1(y1,y2)
        dy2=f2(y1,y2)
    end subroutine
end module
